Camera Raw: Merge to HDR Pro Broken

Merge to HDR Pro was working fine until this morning. I had just successfully finished one bracketed set of a real estate photos and then went on to merge a landscape photo when the Merge function stopped working. The 3 shots opened in the background as usual, but as the last image was being processed all the images Photoshop had loaded disappeared and the Merge Dialog box did not open. I rebooted my computer, no fix. I exported my photos to another format and although the merge dialog box opened this time, the final merge looked awful. All the shadows were completely black with horrible artifacts throughout the image. Suspecting a problem with my source images, I tried to merge the real estate photo that merged successfully and now those images no longer merge properly either.

I googled the problem and found forums with other people running into the same problem, but no fix. One person suggested uninstalling and reinstalling Photoshop CC 2014 as well as Lightroom 5. I did this and HDR is still not working. I've tried everything I can think of and can't find a work around or a fix.

One observation that might be relevant to this issue is this: I run the Merge to HDR Pro twice for each bracketed set and choose a different exposure as the deghosting reference source for each merge. This allows me to take the best deghosting result from highlights and shadows of each merge and combine them to create the highest quality HDR file from of my original source photos. I do this because I delete my source photos and keep the HDR image for archiving. I noticed a lot of instability when running HDR merge multiple times and I have had photoshop unexpectedly quit when working with multiple HDR Merged documents.

True it is ACR that has been updated, however I was alerted to the update to Photoshop CC via the CC Desktop and installed it as a Photoshop CC update (applied also to Lightroom and Bridge). So it was a Photoshop update that was carried out for me :)